Though I've only towed 30 miles to the local GD dealer for some minor maintenance work, it is extremely hilly, up and down, and 70 mph zone.
I kept it at 65, in tow mode, and it was shifting flawlessly in our 2021 F350 DRW crew cab. Matter of fact, it was a little scary because at one point I forgot the 14,000 lbs was back there and found myself hitting 70. A little brake application and I could feel the exhaust brake kick in going down hill (I have it set to auto mode)
Now, coming from a 2000 F350 into the 21st century of new gizmos, it was like sailing Columbus' ship Pinta to a modern day US Destroyer so I was easily impressed.
Ford, Chevy *and maybe dodge* have cut their production lines of HD trucks, as well as light duty due to the chip shortage that apparently won't end this year so you'd better buy soon, if you can find one, and expect the dealers not to be in the dealing mode. Combine that with new corporate taxes going from 22% to 28%, these trucks will not get cheaper anytime soon because those new fed taxes trickle down to the consumer.